Russia and the Syrian military have blamed Israel for a pre-dawn missile attack on a major air base in central Syria that reportedly killed 14 people, including Iranians.
It comes as international condemnation grew over a suspected poison gas attack over the weekend that was blamed on the Syrian government.
Opposition activists said 40 people died in the chemical attack, blaming Syrian government forces.
The UN Security Council planned to hold an emergency meeting to discuss the chemical attack.
Russia’s defence ministry said two Israeli aircraft targeted the T4 air base in Homs province, firing eight missiles.
It said Syria shot down five of them while the other three landed in the western part of the base.
Syrian state TV quoted an unnamed military official as saying that Israeli F-15 warplanes fired several missiles at T4. It gave no further details.
Israel’s foreign ministry had no comment when asked about reports of the air strikes, which did not appear to be related to Saturday night’s alleged chemical weapons attack.
Since 2012, Israel has struck inside Syria more than 100 times, mostly targeting suspected weapons convoys destined for the Lebanese militant group Hezbollah, which has been fighting alongside Syrian government forces.
Most recently, Israel hit the same T4 base in February, after it said an Iranian drone that had violated Israeli airspace took off from the base.
The base, which was used as a launching pad for counter-offensive attacks against Islamic State militants who were at one point stationed close by, is near the Shayrat air base, which was targeted by US missiles last year in response to a chemical weapons attack.
Monday’s missile attack came hours after President Donald Trump warned there would be a “big price to pay” after Saturday’s suspected poison gas attack on the last remaining foothold for Syrian rebels in the eastern suburbs of Damascus.
At least 40 people were killed in that assault, including families found in their homes and shelters, opposition activists and local rescuers said.
Syria’s state news agency SANA initially said the attack on the T4 air base was probably “an American aggression” but Pentagon spokesman Christopher Sherwood quickly denied the US was behind the strike and the agency then dropped the accusation, blaming Israel instead.
SANA said the missile attack resulted in a number of casualties but provided no specific figures.
The Britain-based Syrian Observatory for Human Rights, which monitors the war through a network of activists on the ground, said 14 died, including Iranians and also three Syrian officers.
Rami Abdurrahman, the observatory’s chief, said the assault targeted a mobile air defence unit and some buildings inside the air base.
He added that it also hit posts outside the base used by the Iranians and Iran-backed fighters.
Dmitry Peskov, spokesman for President Vladimir Putin, told reporters on Monday that Israel had not spoken to the president ahead of the air strike even though there may have been Russian military advisers at the base, which he described as “a cause for concern for us”.
The Syrian government denied the chemical weapons allegations, calling them fabrications.
The Russian military said its officers visited the hospital in Douma and talked to the staff, and said they did not confirm reports of the assault.
First responders entering apartments in Douma late on Saturday said they found bodies collapsed on floors, some foaming at the mouth. The opposition’s Syrian Civil Defence rescue organisation said the victims appeared to have suffocated.
The organisation, also known as the White Helmets, and the Syrian American Medical Society, a medical relief organisation, did not identify the substance used but said survivors treated at clinics smelled strongly of chlorine.
Those reports could not be independently verified because of a government blockade around the town.
Hours after the attack, the Army of Islam rebel group agreed to surrender the town and evacuate their fighters to rebel-held northern Syria, Syrian state media reported. The group also agreed to give up its prisoners, a key government demand.
The evacuations follow a pattern of departures around the capital Damascus and other major Syrian cities as the government reasserts its control after seven years of war.
In his tweets on Sunday, Mr Trump called Assad an “animal” and delivered a rare personal criticism of Mr Putin for supporting him.
Mr Trump has declared his intent to withdraw US troops from Syria in the coming months, despite resistance from many of his advisers.
Syrian rescuers from the White Helmets documented 42 fatalities in Saturday’s reported chemical attack but were impeded from searching further by strong odours that caused breathing difficulties, said Siraj Mahmoud, a spokesman for the group.
More than 500 people, mostly women and children, were brought to medical centres complaining of difficulties breathing, foaming at the mouth and burning sensations in the eyes.
Some had bluish skin, a sign of oxygen deprivation, according to a statement, symptoms consistent with chemical exposure.
The observatory gave a higher death toll, saying at least 80 people were killed in Douma, including around 40 who died from suffocation. But it said the suffocations were the result of shelters collapsing on people inside them.
In denying the chemical weapons allegations, a government statement said “the army, which is advancing rapidly and with determination, does not need to use any kind of chemical agents”.
Russia denied any involvement in the attack. Major General Yuri Yevtushenko was quoted by Russian news agencies as saying Russia was prepared to send specialists to Douma to “confirm the fabricated nature” of the reports.
In recent weeks, government forces have recaptured villages and towns in the eastern Ghouta suburbs of the capital. Douma was the only town left holding out.
A 2013 chemical attack in eastern Ghouta that killed hundreds of people was widely blamed on government forces. The US threatened military action but later backed down.
Syria denies ever using chemical weapons during the war and says it eliminated its chemical arsenal under a 2013 agreement brokered by the US and Russia.